Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/112

 water than that underlying it, in which the Selsey fauna lived; and this colder change I associate with the admission of the North Sea through the Dover Strait.

So soon as by these means the elevatory action gained on the tidal erosion, and the Weald was deserted by the sea, the reversal of the drainage of the Stour and Medway into its present direction commenced. Its first result would be that the streams, descending from the northern portion of the Hastings-sand country, and having to seek a new outlet, would form a lacustrine kind of expanse up to the level where a point of outlet along the present line of drainage was found for it. The deposit of that expanse I trace in the wide-spread sheet of gravel which skirts the Medway and its tributaries, the Beult, Eden, and Teise, over the Weald- clay bottom, and which, in the case of the Beult and Teise, is formed almost exclusively of material derived from the Hastings-sand country, locally called " Crowstone gravel," but which in the case of the Eden, as Sir Roderick Murchison has shown*, has a considerable admixture of Lower-Greensand and flint material, and, in the case of the Teise (as I am informed by Mr. Topley), of Tertiary pebbles also, derived, I conceive, from the prior distribution of such material over the area when in the condition shown in sketch map No. III.

A few patches of gravel resting on Weald-clay, but occupying higher ground than this sheet, skirt the Hastings-sand country, such as those at Marden and Wantsuch Green, mentioned by Messrs. Foster and Topley. These are similarly composed of Hastings-sand material ; and though these gentlemen speak of flint having been found in them, it must be excessively rare ; for I could not detect a trace of it. Their age I regard as similar to that of the gravels with Tertiary pebbles about Yalding and at Willesboro' ; but being on the opposite side of the channel formerly occupying the Weald-clay area, they received no Tertiary pebbles like their coeval gravels at Yalding and Willesboro', but were supplied by the material descending from the Hastings-sand hills. The few flint fragments that Messrs. Foster and Topley speak of may have drifted along the island shore from those abundant accumulations of flints described by Sir Roderick Murchison, which are scattered about the more western parts of the Weald. These patches are considerably above the great sheet skirting the Beult, and are divided from it by a slope of bare Weald-clay.

The gravels and brickearths which fringe the valley of the Medway between Maidstone and Chatham, and of the Stour between Ashford and Canterbury, occupying lower levels than the gravels with Tertiary pebbles, already specially discussed, are more difficult to distinguish, as it is obvious that any earlier gravels or brickearths deposited at low levels in these valleys before the drainage was reversed, would, after that event occurred, become undistinguishably mixed up with the deposits from such reversed drainage.

The same remark equally applies to those lowest accumulations of gravel and brickearths which fall within the Thames, the East-


 * Quart. Journ. Geol. Soc. vol. vii. p. 381.